1. Field of the Invention
The present invention relates to a digital wireless communication system for transmitting data between a plurality of remote stations and a master station in the wireless communication, and more particularly to a scramble wireless communication system having a scrambling function for maintaining a privacy of data in wireless communication lines.
2. Description of the Related Art
Data have been conventionally transmitted between a master station and a plurality of remote stations by using wireless communication lines.
FIG. 1 is a view showing an example of a network structure adopting such a conventional scramble communication system, In the drawing, data have been transmitted between a master station (MS) 60 and N (an integer equal to or above one) remote stations (RS) 50.sub.1 to 50.sub.N in a communication system using TDMA (Time Domain Multiple Access) or SCPC (Single Channel Per Carrier) method.
Since an outsider can relatively easily intercept data transmitted on such a wireless communication line, there is employed a privacy function by which the data currently transmitted on the wireless communication line are scrambled as a countermeasure for avoiding the interception.
The description will be given as to a first example of the prior art in which such a scramble process is carried out with respect to the wireless communication line for data transmission with reference to FIG. 2 and 3.
FIG. 2 is a block diagram showing a construction of the master station 60. In the drawing, the master station 60 is constituted by: a switch circuit 22 which receives a data signal (a data signal in the form of, e.g., voice or facsimile) input from a terminal and an information signal (which is referred to as Signal Unit and abbreviated as SU hereinbelow) from a master station control circuit (MASTER CONT) 21 and switches between these signal using a control signal from the MASTER CONT 21; a scrambler 23 which receives output data from the switch circuit 22 and a scramble vector from the MASTER CONT 21 as an initial value of pseudo-random noise pattern (PN pattern) and generates a random pattern to perform scrambling; a transmitter 24 which receives an output from the scrambler 23 and carries out a predetermined digital modulation to transmit data to a remote station by using a predetermined radio frequency; a receiver 25 which receives a transmitted signal scrambled at each of the remote stations 50.sub.1 to 5.sub.N and demodulates a data signal transmitted from the remote station; a descrambler 26 which performs descrambling by receiving demodulated output data which has been scrambled from the receiver 25 and generating the same PN pattern with that of the scrambler 23 and restores the original data; and a switch circuit (SWITCH) 27 which switches between the MASTER CONT 21 and an external terminal to which an output from the descrambler 26 is supplied in accordance with a control signal from the MASTER CONT 21.
On the other hand, FIG. 3 illustrates a structure of one station arbitrarily selected from the remote stations 50.sub.1 to 50.sub.N. As similar to the master station 50, a master station 60 receives a transmitted signal which is obtained by scrambling a data signal input from a terminal through a switch circuit 29, a scrambler 30 and a transmitter 31.
A scramble signal transmitted from the master station 60 is externally output as output data through a receiver 32, a descrambler 33 and a switch circuit 34 or output to a remote station control circuit 28.
When carrying out the communication between the master station 60 and the remote stations 50.sub.1 to 50.sub.N, there is always provided a setup sequence period before transmitting and/or receiving the actual communication data and the information required for the communication (for example, the information of such as radio frequency used in the actual communication) is transmitted between the master station 60 and a remote station 50.sub.k (k is a positive integer arbitrarily selected from 1 to N) at which the communication is desired. The scrambling is not carried out during the setup sequence period in order to simplify the sequence procedure and obtain a reliable communication.
In the master station 60, therefore, the SWITCH 22 performs switching in accordance with the control signal from the MASTER CONT 21 to supply the SU signal to the scrambler 23 during the setup sequence period. The scramble vector which is a parameter for determining an initial value of the scrambler 23 during the setup sequence period is a fixed value (referred to as VO) and the scramble vector of the descrambler 26 is always a VO.
An output signal from the descrambler 26 is switched to the SU side by means of the SWITCH 27 and fed to the MASTER CONT 21.
In addition, as to the remote station 50.sub.x, the REMOTE CONT 28 switches the SWITCH 29 to the SU side and the SU transmitted from the master station is supplied to the scrambler 30. The scramble vector of the scrambler 30 is the VO which constantly gives a fixed value and an output signal from the scrambler 30 is sent to the transmitter 31.
On the receiving side, the scramble vector of the descrambler 33 is the VO and the SWITCH 34 is changed over to the SU side to supply the SU to the REMOTE CONT 28.
The setup operation between the master station 60 and the remote station 50.sub.k is carried out with the above-described settings.
Subsequently, the remote station 50.sub.k randomly generates at the REMOTE CONT 28 a value of the scramble vector (referred to as VF) which is used in the actual communication and transmits the determined value to the master station 60 by using the SU,
Upon receiving a value of the scramble vector VF transmitted by the remote station 50.sub.k, the master station 60 stores this value in the MASTER CONT 21 and determines a value of the scramble vector VF of the scrambler 23 by using the received value. As a result, the scramble vectors which are the same in transmission and reception are obtained, thus enabling the transmitting/receiving processes. That is, the remote station 50.sub.k determines the scramble vector of the descrambler 33 as the VF, and the master station 60 sets the scramble vector of the scrambler 23 as the VF when the SU including the VF is input to the MASTER CONT 21.
Consequently, the data VO having a fixed value are always used as the scra mble vector when transmitting from the remote stations 50.sub.1 to 50.sub.N to the master station 60 in the data communication after the setup sequence period is completed, while the scramble vector VF randomly generated in the remote stations 50.sub.1 to 50.sub.k is used when transmitting from the master station 60 to the remote stations 50.sub.1 to 50.sub.k.
The scramble processing is fixedly carried out when transmitting from the remote stations to the master station while the scramble processing is randomly performed when transmitting from the master station to the remote stations. That is because the structure of the master station which is complicated as compared with those of the remote stations and the enlarged dimension of the antenna lead to the low possibility of interception on the wireless. communication lines between when transmitting from the remote s ations to the master station, thereby lowering the necessity for the scramble processing. Further, since the simplification in the network structure and the improvement of reliability are also intended, these two types of scramble processing are performed.
FIG. 4 shows the flow of signal processing between the remote station 50.sub.k and the master station 60 described above. In the drawing, the scramble vector is set as the VO between the remote station 50.sub.k and the master station during the setup sequence period, and thereafter the scramble vector VF is transmitted from the remote station 50.sub.k to the master station 60. Both the vectors are transmitted by the scramble vector VF in the communication period. In FIG. 4, the fact that the signal transmitted from the remote station 50.sub.k to the master station 60 is represented as the SU (VO) means that the SU signal is transmitted with the scramble ector which is the VO. The same representation is used hereinbelow. The scramble vector VF is randomly generated from the ma ter station to the remote station in the prior art structure described above. The parameter value of this scramble vector VF is transmitted from the remote station to the master station during the setup sequence and the actual communication periods, but alternatively the scramble vector and the communication data may be subjected to the frequency division to be transmitted.
FIG. 5 is a block diagram showing a remote station used in such a second example of the prior art.
In this example, it is basically assumed that the transmission is carried out between remote stations (the master station may be included therebetween).
In the drawing, a scramble vector V randomly generated by each remote station at a remote station control circuit (REMOTE CONT) 35 is set in a scrambler 36 and input data (for example, data of such as voice or facsimile) is scrambled. The scramble vector used here is encoded into a VZ by an encoder circuit 39 and input to a multiplexer circuit 37.
The multiplexer circuit 37 subjects the input data and the scramble vector to the frequency multiplexing in the different frequency bands and transmits them by a transmitter circuit 38.
In the receiving portion, a signal received by a receiver circuit 40 is separated from a main sound signal and an encoded signal VZ of the scramble vector by means of a separator circuit 41, and the main sound signal is input to a descrambler 42 while the encoded signal VZ is decoded into the scramble vector V set on the transmitter side by a decoder circuit 43 to be supplied to the remote station control circuit 35 and the descrambler 42. The descrambler 42 performs descrambling by using this scramble vector V and outputs a sound signal. Such a scrambling method is disclosed in, for example, Japanese patent laid-open publication No. Hei 4-192638, 1992.
In the configuration of the above-described prior art, the scramble vector transmitted from the master station to the remote stations is randomly generated for each communication in the first example, but the scramble vector transmitted from the remote stations to the master station is always the same even when the remote station with which the master station communicates varies, and hence there occurs such a problem that the scramble vector can be decoded by intercepting the communication from every remote station.
Further, although the scramble vector which is random in the bidirectional manner can be used when communicating between remote stations as described in the above second example, the transmission frequencies having multi-value parameters are excessively used outside the band, thus reducing the frequency efficiency in all the communication lines.